Exchangeable zinc pool size at birth is smaller in small-for-gestational-age than in appropriate-for-gestational-age preterm infants

Background: Small-for-gestational-age (SGA) infants are susceptible to postnatal zinc deficiency, but whether this susceptibility is due to intrauterine factors or to high postnatal growth requirements is unknown. Objective: We hypothesized that the size of the exchangeable zinc pool (EZP), which reflects metabolically available zinc, would be smaller in SGA than in appropriate-for-gestational-age (AGA) infants born prematurely. Design: Intravenous Zn-70 (45 mu g/kg) was administered to 10 SGA infants (8 boys) with a mean (+/- SD) gestational age of 33.3 +/- 1.8 wk and to 11 AGA infants (8 boys) with a mean (+/- SD) gestational age of 32.4 +/- 1.2 wk within 24 h of birth. The EZP was determined from isotope enrichment in spot urine collections on days 3-7. Results: The mean birth weight of the SGA infants was 1.30 +/- 0.2 kg and of the AGA infants was 1.84 +/- 0.3 kg (P = 0.0001). The EZP size was significantly smaller in the SGA than in the AGA infants on an absolute basis (13.3 +/- 2.8 and 25.2 +/- 8.1 mg; P = 0.0002) and relative to body weight (10.3 +/- 2.5 and 13.9 +/- 4.5 mg/kg; P = 0.02). The difference remained significant after adjustment for gestational age and birth weight. Conclusion: These data provide evidence for differential zinc status at birth between SGA and AGA infants born prematurely at similar stages of gestation and offer at least a partial explanation for the reported benefits of postnatal zinc supplementation.